Laser sensors based on self-mixing interferometry generally provide the possibility of measuring velocities, vibrations and distances and thus cover a broad range of applications. SMI laser sensors make use of the effect that laser radiation which is scattered or reflected back from a target object and re-enters the laser cavity, interferes with the resonating radiation and thus influences the output properties of the laser. The resulting variations in output power or frequency contain traceable information on the movement or the distance of the target object with respect to the sensor. The laser output signal, which contains this information, is typically collected via a photodiode. A movement of the target object, for example, causes a Doppler shift of the backscattered laser radiation which interferes with the laser field in the laser cavity, resulting in a modulation of the intracavity field and output power. The strength of the modulation depends on the strength of the backscattered laser radiation. The operation principle of laser self-mixing is explained, for example, in G. Giuliani et al., “Laser diode self-mixing technique for sensing applications”, Journal of Optics A: Pure and applied optics 4 (2002), pages S283 to S294.
The strength of the backscattered laser radiation depends on the laser power of the laser and on the reflection properties of the target object. When the strength of the backscattered laser radiation is low, the noise present in the laser radiation dominates and the modulation representing the SMI signal is no longer observable.